The present invention relates to devices and methods for stabilizing the spine. It more particularly relates to stabilization of a spinal column that has been rendered unstable by any disease such as a tumor, infection, or congenital anomalies affecting the vertebral bodies or associated supporting structures.
Preservation of neurologic function and relief of pain are the primary goals of treatment of diseases affecting the vertebral bodies. Oftentimes such diseases themselves destroy the vertebral body or the surgical treatment necessitates its resection. As a result of the loss of bony support, the vertebral column is unstable causing pain, progressive neurologic deterioration, and physical deformity of the back. In such cases stabilization of the spinal column by an anterior surgical approach may be required.
A common technique utilized for stabilization of the anterior spine is to fill the space of the resected vertebral bodies with methylmethacrylate or other similar plastic or plastic-like polymers. Difficulties encountered with this method include the inability to bond securely the plastic material to tissue or bone, the injurious release of heat into the surrounding tissue because of the exothermic chemical reaction required for polymerization of the plastic, potential stress fractures of the plastic caused by mechanical forces related to weight bearing, and the need to mold the plastic to the appropriate size which is often technically cumbersome and difficult. Although metallic pins or struts are often used to reinforce the methylmethacrylate, they do not provide adequate mechanical or torsional stability.
Attempts at spinal stabilization have also been performed utilizing a variety of metal pins and struts as mentioned previously. One common method is to penetrate adjacent vertebral bodies with Steinmann pins and then surround the pins with methylmethacrylate. Many technical problems are associated with this procedure including (1) those difficulties specially related to methylmethacrylate; (2) the inability to maintain distraction adequately while polymerization of the plastic is taking place; (3) the inability to adequately place the pins because of space restrictions; and (4) the fracture of the pins resulting in loss of stability and possible movements of the methylmethacrylate.
Another common method of anterior spine stabilization utilizes bone to act as a replacement for the vertebral body after its removal. Bone of the iliac crest, fibula, or tissue-banked cadaver bone has been used. Typically, the bone material is introduced into a hollow place holder or "cage" positioned between adjacent vertebrae to be fused. Reference is made, for example, to U.S. Pat. No. 4,820,305 entitled PLACE HOLDER, IN PARTICULAR FOR A VERTEBRA BODY and issued to Harms et al on Apr. 11, 1989. In that patent, there is disclosed a mesh-like cage having a diameter and axial length selected so that the cage fits between the remaining vertebrae and maintains the original separation therebetween. The ends of the cage define teeth which are intended to engage the end plates of the superior and inferior vertebral bodies and reduce the effects of torsion. Several limitations of this technique exist, however. Immediate stabilization is not possible because adequate bony fusion requires three to six months. When radiations therapy and/or chemotherapy are required to treat the primary disease process affecting the spine, the implanted bone graft often does not survive and this results in further spinal instability secondary to resorption of the bone graft.
Yet another method for anterior spinal stabilization utilizes one or two metal rods attached by screws to the lateral aspect of the adjacent vertebral bodies. This procedure was first described by Dunn in 1980. There are two primary limitations of this procedure. First, it cannot be used in certain areas of the spine because of the vital structures adjacent to the lateral aspect of the vertebral body. Second, the device is inherently weak since mechanical stresses are maximally exerted at the site of attachment of the device to the vertebral body by small metal screws.
Metal rod distraction utilizing Harrington or Knodt rods, with or without methylmethacrylate, is another method to stabilize the spine via the anterior approach. This procedure was described by Harrington in 1976. One or more rods are typically attached via hooks to the anterior portion of the vertebral body and they can then be covered with methylmethacrylate to further secure them in place. The major disadvantages of this technique is the slippage of the stabilizing rods secondary to the mechanical forces generated by spine movement. These forces are greatest at the site of hook insertion and may cause fracture of the vertebral body to which it is attached. Moreover, involvement of the adjacent vertebral bodies with disease may weaken the bone at the site of hook insertion and lead to migration of the rods with resultant collapse of the spinal column.
Of the drawbacks discussed in connection with the above techniques, perhaps the most inconvenient for the patient is the reliance upon external fixation for the extended periods of time required for vertebral fusion. As such, more recent efforts have been directed to the development of inert prosthetic implants to replace the resected vertebral body and adjacent discs. In U.S. Pat. No. 4,932,975 entitled VERTEBRAL PROSTHESIS and issued to Main et al on Jun. 12, 1990, for example, there is described an implant which comprises a pair of rigid housings joined by a connecting structure that is operable to shift the housings apart into supporting engagement with the healthy vertebral bodies adjacent the resected vertebral body. Anchoring pins project from each respective housing and into the end plate of a corresponding vertebral body. A primary disadvantage of the implant described by Main et al, however, is that it does not adequately provide for bone ingrowth and relies substantially on the anchoring pins to maintain the prosthesis in position.
Reference is also made to U.S. Pat. No. 5,062,850 to McMillan et al entitled AXIALLY-FIXED VERTEBRAL BODY PROSTHESIS AND METHOD OF FIXATION and issued on Nov. 5, 1991. McMillan et al disclose a vertebral body prosthetic device which comprises upper and lower end plates separated by a plurality of support columns or posts. The plates are affixed to neighboring vertebral bodies via axially oriented screws. Bone is packed between the plates and across the sides of the device after it has been fixed into position to facilitate eventual bone ingrowth. One drawback of the McMillan et al device is that if the bone material placed within the device loosens, it may become displaced and irritate adjacent tissue. Moreover, like the device taught by Main et al, the McMillan et al device relies largely upon the axial screws to provide fixation of the implant. Instead of extending axially between the entire area of the vertebral body end surfaces, bone ingrowth is limited to gradual encasing of the implant.
In U.S. Pat. No. 5,192,327 entitled SURGICAL PROSTHETIC IMPLANT FOR VERTEBRAE and issued to Brantigan on Mar. 9, 1993, there is disclosed yet another vertebral body prosthesis. Brantigan discloses a prosthetic implant having an oval cross section and comprised of a plurality of modular segments. The edges of the segments have grooves such that individual sections may be interdigitated to achieve a desired length. The individual sections are held together by a bar member that extends axially within the interior bore of each segment and that divides the interior bore into two central apertures. Unlike the prostheses disclosed by Main et al and McMillan et al, the Brantigan prosthesis, does provide for the eventual uninterrupted, axial ingrowth of bone between the end plates of the vertebra surrounding, the resected vertebral body. However, Brantigan relies solely on the peripheral edges of the hollow implant to provide the initial fixation. As such, the device is initially vulnerable to torsional displacement or lateral shifting.
Accordingly, the principal object of the present invention is to provide a novel device and method for stabilizing the spinal column following the resection of a vertebral body and adjacent discs but without the shortcomings and disadvantages of the prior methods as set forth above.